(1) Field of the Invention
The present invention relates to plethysmographs used to measure changes in air volume, such as in non-invasive pulmonary testing of small animals, and in particular to larger plethysmographs that provide a test chamber with an airtight seal without the use of clamps, but which can be readily opened when desired. The invention further relates to a reduced noise plethysmograph that include means for air sampling.
(2) Description of the Prior Art
Plethysmographs are used in research to collect data relating to changes in air pressure within a test chamber. An example of such data is pulmonary data from small animals, such as mice. Most plethysmographs are comprised of a test chamber to enclose the test subject, a reference chamber, and a differential pressure transducer connected to the two chambers, e.g., via tubing extending from a port in each chamber to the transducer. Both chambers are in communication with the ambient air, i.e., the air within the room where the tests are being conducted, through restricted airflow openings, or pneumotachs.
As changes to the air volume within the test chamber occur, pressure variations are recorded by the transducer, which normally displays the recorded data in numerical form or as a graph. Air pressure within the test chamber can also vary due to changes in the pressure of air entering the test chamber through the pneumotachs. This non-chamber originated air pressure variation, known as background noise or simply noise, can adversely affect the accuracy of the recorded data, since the transducer also measures the noise.
Plethysmographs are commonly used to measure the pulmonary activities of test animals that are completely or substantially enclosed within the test chamber. As the test animal inhales or exhales, the changes in air volume results in pressure variations that are recorded by the transducer.
The purpose of the reference chamber is to partially reduce the noise affect. A second tube extends from a reference chamber outlet to the transducer. The transducer simultaneously measures variations in air pressures within the two chambers, and subtracts the reference chamber measurements from the animal chamber measurements. As a result, the net pressure variations are essentially attributable to the respiration patterns of the test animal. Preferably, the test and reference chamber pneumotachs are close to each other to minimize variations in exterior air patterns.
A representative plethysmograph of the type used to measure small animal pulmonary responses is shown and described in commonly assigned U.S. Pat. No. 5,379,777 to Lomask, the entire disclosure of the patent being incorporated herein by reference.
U.S. Pat. No. 6,902,532 to Lomask, the entire disclosure of this patent also being incorporated herein by reference, describes an apparatus for further reducing background noise. In accordance with the disclosure of the '532 patent, a noise-reduction manifold is provided to direct exterior air from a common opening into both the reference and test chambers. Preferably, the distance from the exterior air opening to the two chambers is approximately the same, so that any changes in air entering the manifold will be equally experienced by both chambers. As a result, subtraction of reference chamber pressures from test chamber pressures provides essentially a noise-free measurement of pressure changes within the test chamber that is attributable to the test subject.
However, further improvements in plethysmographs of the type described in the above patents are still desired, especially in plethysmographs sized to accommodate larger test subjects. In particular, there is a need for a plethysmograph having a reference chamber that is attachable to a test chamber in a manner that will result in an airtight enclosure of the test chamber without the use of latches or clamps, while still enabling the operator to quickly and easily open the test chamber when desired. There is also a need for a reduced noise plethysmograph that includes a means for quickly and accurately sampling the air that is entering the test chamber.